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CHAPTER 12 



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I [GURE I 2 8. Salivary gland 

 chromosomes heterozygous for a 

 shift within the right arm of chro- 

 mosome 3 of Drosophila melano- 

 gaster. A piece from map region 

 "98" is inserted into map region 

 "91." I lie rightmost buckle is 

 clue to the absence of the shifted 

 segment; the leftmost buckle is 

 due to its presence. ( Courtesy of 

 B. P. Kaufmann. ) 



is called a shift. If, in the heterozygote for 

 a shift, the homologs pair up and a crossing 

 over occurs in the region of the shift, a sec- 

 tion of one of the crossovers will be in 

 duplicate, as can be seen by tracing the re- 

 sultant strands. 



Two breaks in one chromosome and one 

 in a nonhomolog can result in the interstitial 

 piece of the first chromosome being inserted 

 into the second. This result is called trans- 

 position. A transposition-containing chro- 

 mosome can occur in subsequent generations 

 not with the nonhomologous, deficient chro- 

 mosome from which the piece was trans- 

 posed, but with two normal chromosomes 

 of that type. In this way an individual is 

 produced containing a pair of normal hom- 

 ologs and a part of the normal homolog 

 present in hyperploid condition in a non- 

 homolog. 



The preceding indicates how the same 

 type of structural change — duplication — can 

 result from different types of breakage 

 events. For this reason, one cannot always 

 specify the particular number of nonrestitut- 

 ing breaks originally involved by observing 

 the resultant rearrangement and, therefore, 

 the explanation proposed is always the sim- 



plest one. Note also that loss of an entire 

 chromosome can occur after breakage; thus, 

 not all such losses come from nondisjunc- 

 tion. Contrary to nondisjunction, however, 

 breakage events cannot produce trisomies. 



Cytogenetic Detection of Structural Changes 



The question of how structural changes in 

 chromosomes are detected may have arisen 

 during the preceding discussions. Such mu- 

 tants may be detected initially by cytological 

 examination, or they may be noted first by 

 their effects on the phenotype when genetic 

 tests are made. Thus, detection and identi- 

 fication of structural changes can be made 

 cytologically, or genetically, or by a com- 

 bination of both methods. 



When heterozygous, deficiencies can some- 

 times be recognized genetically since they 

 permit the expression of all genes which are 

 hemizygous in the nondeficient chromosome. 

 Inversions and translocations can be sus- 

 pected when mutant heterozygotes show a 

 marked reduction in offspring carrying cross- 

 overs. Using appropriate genetic markers, 

 inversion homozygotes show some genes in 

 the reverse of normal order, whereas in 

 heterozygotes or homozygotes for transloca- 



